A sequence of steps for tanning animal hides and skins such as cowhide is roughly divided into three steps: preparative step, tanning step and finishing step. The preparative step generally comprises soaking a raw hide in water, fleshing, unhairing and liming, splitting, scudding, re-liming, deliming and bating although slightly different depending on the kind of leather to be produced, the kind of tanning step subsequent to the preparative step, and the like.
Currently, the unhairing and liming operation in leather tanning are continuously carried out in a drum or a paddle using concentrated calcium hydroxide and sulfide as unhairing agent to reduce the operation time and to shorten the operations (hair-burn method). More specifically, the hide is softened on absorption of water in the water-soaking operation, and is immersed in milk of lime containing sodium sulfide or sodium hydrogensulfide serving as an unhairing accelerator. As a result, the epidermal tissue is decomposed and separated from the corium, and the hair root is loosened to facilitate unhairing while the hairs are decomposed and dissolved due to the action of the unhairing accelerator.
However, this method permits decomposition and dissolution of hairs, making it impossible to recover the hairs, so that the waste water invariably contains a large amount of keratin decomposition products derived from the dissolved hairs together with concentrated calcium hydroxide and sulfide, which would be likely to cause significant environmental pollution. Further, this involves a serious drawback that the disposal of the waste water necessitates large-scale facilities and incurs high costs.
In recent years, in order to overcome the drawback encountered in the hair-burn method and to reduce the pollution load of waste water, a hair-saving unhairing method was proposed which is intended, based on the hair-burn method, to protect the hair body portion against the decomposition occurring due to the unhairing agent. Typical examples of the proposed methods include BLAIR Method [Leather, 1998 (Feb.):23-26 (1988)] and SIROLIME Method [Cranstone, R. W., Davis, M. H., Scroggie, J. G., J.S.L.T.C., 70, 50-55 (1986)].
However, even these proposed methods are still unsatisfactory in the effect of reducing the pollution load of waste water. Further, the SIROLIME Method has a shortcoming that hydrogen sulfide gas and chlorine gas are given off from sodium hydrogensulfide and hypochlorite used, necessitating their disposal and posing a problem of adversely affecting the leather surface, which referred to as silver surface, to give a defect.